Photonic technology can be used to enhance the measurement and distribution of microwave signals. This field is sometimes called radio-frequency (RF) or microwave photonics. As an example, antenna remoting is a term that refers to modulating a microwave or RF signal onto an optical signal. The modulated optical signal can then be sent over long distances via a fiber optic cable (an optical channel) which has lower loss, weight, and cost than a high speed RF cable and does not suffer from electro-magnetic interference, to a receiver that converts the signal back into the electrical domain. Ideally very simple equipment is located at the antenna as the antenna location is chosen for optimal reception and often there is little room for additional components near the antenna.
Ideally the microwave signal is exactly reproduced at the receiver with no loss of signal integrity. However, often times the signal integrity is limited by nonlinearities in the optical modulator or the optical demodulator at the receiver. This creates spurious signals whose magnitude depends on the magnitude of the input RF signal, thereby limiting the dynamic range of operation. This effect is sometimes characterized by the spurious free dynamic range (SFDR) metric. Methods to linearize modulators are often complex, and one does not want complex equipment that may be sensitive to drift or parameter settings at the antenna. A method to cancel out the third order nonlinear distortion that uses the natural modulation efficiency (measured by the voltage required to induce a π phase shift in the modulator, or Vπ) difference of an optical modulator between two different optical wavelengths or the difference between two polarization axes of an optical modulator have been demonstrated. While effective in cancelling out nonlinear distortions it turns out such methods also reduce the gain of the systems which in turn reduces the noise figure. The gain reductions of linearized systems can be substantial and are commonly in the (10-20) dB range. This greatly reduces the utility of such linearization methods.
In addition to SFDR, other important system metrics are gain and noise figure (NF). Gain is the ratio between the power of RF signal received to the power of the RF signal applied at the optical modulator. NF is related to the amount of added noise the measurement system produces. If a system has a linear gain of g, the input signal has noise of nin, and the measurement/distribution apparatus adds noise added of nadd, then the noise out of the device can be written as nout=g·nin+nadd. Using this notation NF=10 log (1+nadd/g·nin). We see that all other things being equal, higher gain also benefits NF. We see now why cancelling out distortions to improve SFDR may cause other problems, specifically reduced gain and increased NF. Gain can be increased, for instance, by increasing the modulation efficiency (reducing the Vπ) of the modulator.
When a typical phase modulator is used then the applied phase shift of the modulator (the signal phase modulation depth) is over some range linearly proportional to the RF signal voltage applied at the modulator (VIN). That is ϕ=p·VIN, where ϕ is the instantaneous optical phase shift, and p is a phase modulation efficiency constant where p=π/Vπ and Vπ is the amount of voltage required to generate a π phase shift in the modulator. Typically gain is proportional to p2, or equivalently ϕ2, thus if Vπ is reduced by a factor of 2 then gain increases by a factor of 4 (6 dB). Increasing p, or equivalently reducing Vπ, increases gain and is thus desirable.
Another method of linearization employs optical nonlinearity in an optical material having third-order nonlinearity (a centrosymmetric material; e.g. a nonlinear optical fiber) has also been demonstrated, but despite the potential power of the technique it required very high optical powers, long lengths of fiber, and tends to add noise. The high required power is due to the low magnitude of the third order nonlinearity and the added noise comes in part from the mixing process between two or more lasers (one laser being the signal and the other being the pump, where the strong pump power helps to generate the desired nonlinear effect), and where the mixing process may be cascaded thereby also cascading the noise.
Another benefit of RF-photonics can be all optical down-conversion, which in principle can replace the electrical mixers more commonly used to down-convert a very high microwave carrier frequency to a lower and more easily detectable carrier frequency. The signal bandwidth of B (in Hz) remains intact, but the carrier frequency about which the signal bandwidth is centered is reduced which makes detection and subsequent processing easier. The more typical solution is to detect the signal at the carrier frequency then down-convert it using an electrical mixer. Electrical mixers add loss, add distortions, and often have less operating frequency range than desired.
What is needed is a system or method to increase the gain of microwave photonic systems. The gain improvement can be used to linearize photonic links without substantially reducing NF. The system should be compatible with implementations that are simple at the transmitter (the initial microwave-to-optical modulator) and with advanced techniques such as all optical down-conversion. These advantages should come with minimal drawbacks.